An External ID provides the correct mechanism for matching Salesforce records with identifiers maintained by another application. It is a custom field marked with the External ID attribute and typically stores the unique identifier assigned to a record by a core banking system, ERP platform, policy system, loan-origination system, or another external data source.

Salesforce import tools can use External IDs to identify existing records. When a record in an incoming file contains an External ID that matches an existing Salesforce record, the import process can treat the records as corresponding records rather than blindly inserting another copy.

External IDs are also fundamental to upsert operations. The integration supplies the External ID, Salesforce searches for a matching value, and the operation updates the matching record or creates a new record when no match exists. For the strongest duplicate control, the field can additionally be configured as Unique, ensuring that the same external identifier cannot be stored on multiple records.

User ID, Org ID, and Data Load ID do not provide the required source-system record-matching mechanism. External ID is therefore the appropriate integration identifier.